The normal correlation has the Fourier structure [1]},
\(\langle B_i(\vec{x})B_j(\vec{x}\,^{\prime }) \rangle = \int \frac{d^3k}{(2\pi )^3} \frac{\delta _{ij}-\hat{k}_i \hat{k}_j}{2}P_B(k)e^{i\vec{k}\cdot (\vec{x} - \vec{x}\,^{\prime })},\nonumber \\\) 
